1. Field of the Invention
The present invention relates in general to image pickup apparatus and, particularly, to a control system for automatically controlling the location and level of the scanning electron beam in such image pickup apparatus.
2. Description of the Prior Art
Image pickup tubes typically involve three major subassemblies, namely, the image section, the storage target section, and the scan section. The image section employs a photo-emissive surface and electron optics to convert an optical image into an electron image, which is then focused upon the surface of the storage target and creates a corresponding electric-charge image. The storage target integrates or stores the focused electric charge before readout and erasure by a scanning electron beam generated in the scan section. The low velocity scanning electron beam that is generated by the cathode repetitively scans the rear surface of the storage target, which corresponds to the anode, to generate a time-varying electric signal that is proportional to the magnitude of the spatial charge distribution produced by the image focused onto the front of the storage target, that is, the anode. The scanning electron beam currents in the image pickup tube serve to lower the potential created by a highlight of an optical image focused onto the storage target down to what is essentially the cathode potential. That is, the beam currents act to supply a beam of electrons which effectively neutralizes the positive charges produced on the photoelectric transducer surface, as embodied by the storage target surface. The level of the scanning electron beam current must be chosen to be sufficiently high, otherwise the scanning beam electrons will be insufficient to neutralize the charge caused by a highlight of the optical image. That is, unless the scanning electron beam has a high enough concentration of electrons it will not be sufficient to reduce the potential caused by the image highlight on the storage target to approximately the cathode level. Simply increasing the power of the beam of electrons is an unacceptable solution to this problem, since in a photoconductive image pickup tube as the beam of electrons is increased the focus of the beam is decreased and, thus, the beam falling on the storage target will be out of focus, resulting in reduced resolution. This problem is more acute in frequency-separation or phase-separation single-tube color image pickup apparatus, since the out-of-focus beam condition adversely affects color saturation, color uniformity, and the signal-to-noise ratio (S/N) of the color signals.
One proposed solution to eliminate any electron shortage in the beam scanning such highlights and to prevent deterioration of the color image quality has been to employ an automatic beam control circuit that would act to control the level of the electron beam in response to the brightness of the object to be televised, that is, the image being detected. Such proposed automatic control circuit produces an output signal current from the storage target in the image pickup tube and feeds such output signal, together with a predetermined DC voltage level, back to the first grid of the image pickup tube through means of a feedback circuit. In this system, when the output current from the image pickup tube exceeds the predetermined DC voltage level, a control voltage from the feedback circuit is added to the grid voltage, thereby increasing the beam current and avoiding any electron shortage in the scanning beam.
Nevertheless, in using this above-described automatic beam control circuit, it has been found that it tends to oscillate quite easily, and it is difficult to adjust the circuit so that both beam electron shortage and oscillation can be prevented. The cause of this oscillation has been attributed to the positive feedback loop that is present when beam electron shortage occurs.
One proposed solution to prevent this unacceptable oscillation and to permit the automatic beam control circuit to operate correctly is to require the characteristic curve representative of variations of the first grid voltage versus variations of the beam current and the characteristic curve indicative of variations of the signal current obtained through the automatic beam control circuit versus variations of the first grid voltage to be quite similar. This characteristic curve similarity requirement has proven to be difficult to meet because of the inherent circuit design, the required adjustments, and time dependency changes of the image pickup tube. Additionally, a previously proposed method to prevent these oscillations by using negative feedback of a cathode current has not proven to be effective, since the characteristic curve representative of the first grid voltage versus the cathode current is not sufficiently similar to the characteristic curve representing the first grid voltage versus the beam current. Therefore, while a solution to preventing degradation of the color image signal caused by deficiencies in the scanning beam electrons has been known, such solution causes almost as many problems as it solves.